J. A. BASSHAM AND M. CALVIN 



but upon turning on the light some ATP will be formed in the 

 chloroplast, owing to the oxidation (back reaction) of some of the 

 photochemically generated reducing power by O2 liberated from 

 photosynthesis, or by an intermediate oxidant. The formation of 

 ATP by the oxidation of an intermediate reductant (i.e., 

 TPNH) is known from studies of oxidative phosphorylation. 

 That some of the ATP was formed during the transfer of the 

 electrons between intermediate reductant (TPNH) and inter- 

 mediate oxidant (i.e., metalloflavin and/or cytochromes) is also 

 an intergrai part of this process, although ultimately the electrons 

 ([H]) are transferred to molecular O2. The experimental 

 observation of the formation of ATP by the recombination of 

 photo-produced intermediate reductants and oxidants without 

 the possibility of the intervention of molecular oxygen has been 

 made by Frenkel (28) using the plastids isolated from purple 

 bacteria which are incapable of making molecular O2. A simi- 

 lar observation has been made by Arnon and co-workers (2) 

 using specially prepared whole chloroplasts from spinach. In 

 the latter case it was necessary to add other redox systems to 

 replace molecular O2, such as the naphthoquinone related to 

 vitamin K, or ascorbic acid. In addition some participation 

 of added FMN was demonstrated. 



In regard to the location of the processes of photosynthesis, 

 then, the chloroplast or its immediate environs seems to be the 

 unique location of all important parts and may provide a con- 

 tainer or matrix for the complex system of enzymes and cofactors 

 involved in the carbon reduction cycle as well as a support for 

 the pigment and enzyme structure which carries out energy 

 conversion and decomposition of water. 



The Carbon- Reducing Enzymes 



Two powerful tools have been brought to bear on the problem 

 of the path of carbon dioxide reduction during photosynthesis 

 during the past decade : tracer elements and paper chromatog- 

 raphy. It is unlikely that our present degree of knowledge of 



34 



